1. Field of the Invention
The present invention relates generally to a scanning tunneling microscope, and more particularly to a scanning tunneling microscope having an observation optical system for optically observing an object.
2. Description of the Related Art
A scanning tunneling microscope (STM) was proposed by Binnig et al. in 1982 in U.S. Pat. No. 4,343,993. A sharply pointed probe is approached to the surface of an object, and a bias voltage is applied between the probe and the object. Consequently, a tunnel current flows between the probe and the object. It is known that the distance between the probe and the object varies by about one Angstrom, and that the tunnel current varies by about one order of magnitude. The STM utilizes the property of the tunnel current, thereby measuring the surface configuration of the object in an atomic level. For example, the probe is scanned across the surface of the object, while the distance between the object and the probe is servo-controlled by a piezoelectric element to keep the value of the tunnel current constant. The voltage of a servo control signal is recorded to obtain an image reflecting the surface configuration of the object in an Angstrom level.
The resolution of the STM is very high. Thus, the STM is not suitable for object observation with low magnifying power, for selecting an observation region. Due to this situation, there has been proposed an STM in which an optical microscope is built (hereinafter, called "optical microscope-built-in STM"). This type of STM includes an observation optical system capable of optically observing the surface of the object simultaneously with the STM measurement.
In an optical microscope-built-in STM, a probe supported by an optically transparent support member is situated between an objective lens and an object. In the optical microscope built-in STM, optical object observation is performed prior to STM object observation. A target location for STM observation (using a probe) is recognized in the form of the shadow of the probe within an optical microscopic visual field. The shadow of the probe interrupts the optical observation. In addition, when the surface of the object is optically observed in the stage wherein the optical microscopic focal point is close to the tip of the probe, the tip of the probe may contact the object owing to the unevenness of the surface of the object, and the probe may be damaged.
Another problem of the STM resides in the electrode for applying electric power to an object. In the STM, an electrode must be provided on the object, in order to apply a bias voltage across the object and the probe and to detect a tunnel current flowing across the object and the probe. The electrode must be replaced each time the object is changed. In order to omit the replacement step, an object table for placing the object thereon has conventionally been used as an electrode, whereby a bias voltage is applied through the object table. However, since the area of the object table (or the electrode) is large, the influence due to external electromagnetic noise is serious, and a detection value of a tunnel current is adversely affected by the noise. As a result, it is difficult to obtain an exact STM image reflecting the surface of the object.
Still another problem of the STM is the leakage of current to a probe. It is common practice that the probe of the STM is driven by a piezoelectric driving element. Since a high voltage is applied to the driving electrode of the piezoelectric driving element, current leaks from the driving electrode to the probe. Thus, a noise component is contained in the detected tunnel current.